Blender to Unity: mesh disappears when updating blend file
It looks like the original name of that mesh was changed at data level (3 vertex triangle tab on properties name is differenr than tab where cube is) on Blender.
Second reason, could be because the prefab is searching for a different hierarchy order. Thus, it breaks it (un parents it because of bone name or mesh name at data level).

BFCT_Schiller